Tokyo -LRB- CNN -RRB- -- Japan began dumping thousands of tons of radioactive water into the Pacific Ocean on Monday , an emergency move officials said was needed to curtail a worse leak from the crippled Fukushima Daiichi nuclear power plant .

In all , about 11,500 tons of radioactive water that has collected at the nuclear facility will be dumped into the sea , officials said Monday , as workers also try to deal with a crack that has been a conduit for contamination .

The radiation levels were highest in the water that was being drained from reactor No. 6 , the officials said .

These are the latest but hardly the only challenges facing workers at the embattled power plant and its six reactors , which have been in constant crisis since last month 's ruinous earthquake and tsunami .

Officials with Tokyo Electric Power Company , which runs the plant , proposed the release of excess water that has pooled in and around the Nos. 5 and 6 reactors into the sea . But most of the dumped water -- 10,000 tons -- will come from the plant 's central waste treatment facility , which will then be used to store highly radioactive water from the No. 2 unit , an official with the power company said .

The water in reactors Nos. 5 and 6 is coming from a subdrain and was n't inside the building itself , officials said . Tests suggest that groundwater is the source of the contamination in these two units , but they are not certain .

Japanese Chief Cabinet Secretary Yukio Edano called the dumping `` unavoidable . '' The liquid was most likely contaminated in the process of trying to cool nuclear fuel rods .

The scope of the dump was staggering .

`` For an idea about how much is 11,500 tons , one metric ton is 1,000 kilograms or about 2,200 pounds , which is close to an English ton . Water is about 8.5 pounds per gallon , so one ton is about 260 gallons , '' said Gary Was , a professor of nuclear engineering at the University of Michigan . `` So 11,500 tons is about 3 million gallons . A spent fuel pool holds around 300,000 gallons . So this amount of water is equivalent to the volume of roughly 10 -LRB- spent fuel pools -RRB- . ''

It could take 50 hours to dump all the water , Tokyo Electric said .

The dumping of so much radioactive water into the ocean conjures up fears of mutated sea life and contamination of the human food chain , but experts said the radiation will be quickly diluted , minimizing risk .

`` To put this in perspective , the Pacific Ocean holds about 300 trillion swimming pools full of water and they 're going to release about five swimming pools full of water . So hopefully the churning of the ocean and the currents will quickly disperse this so that it gets to very dilute concentrations relatively quickly , '' said Timothy Jorgensen , chair of the radiation safety committee at Georgetown University Medical Center .

`` It 's a considerable amount of water , but the immensity of the Pacific Ocean will quickly dilute this amount of water to harmless levels , '' he added .

John Till , president of Risk Assessment Corp. , similarly cited the vastness of the ocean in helping to minimize harm , and said he does not expect to see any permanent effects on marine life , even close in to the plant .

However , he said officials should continue to monitor the radiation levels closely .

`` What we have to watch is how these materials accumulate in food products and then could be consumed by people , '' Till said .

The build-up of water could cause problems around the nuclear facility , which is 240 kilometers -LRB- 150 miles -RRB- north of Tokyo , Edano said Monday .

Authorities have made a priority of dealing with water from the No. 2 unit , some of which has been gushing into the sea through a crack in a concrete shaft .

`` The radioactivity level is very high near the No. 2 reactor , and we know this . We have to stop the leak as early as possible to prevent this from going into the sea , '' Edano said . `` The radioactivity level is much less in the water from the Nos. 3 and 4 units . ''

Nuclear and Industrial Safety Agency officials said Monday night that the hope is that pumping out the No. 2 reactor turbine plant will lower the water level enough that contaminated liquid wo n't be able to reach the sea .

`` I am not able to say for certain whether or not this will be the last discharge , but we certainly would like to avoid releasing any such water into the sea as much as possible , '' agency spokesman Hidehiko Nishiyama said .

Officials were still awaiting test results to confirm the water pouring into the ocean is leaking from the highly radioactive No. 2 reactor .

`` We do n't know clearly , but we feel it is somehow leaking from Unit 2 , '' Nishiyama said . Even if the water is confirmed to have come from the reactor , neither Tokyo Electric nor government officials know how it is making its way from the reactor to the leaking pit , he said .

Once the water is pumped out of the waste treatment reservoir , the agency believes it can safely transfer the water from the basement of the No. 2 turbine plant to the reservoir without further leaks , he said .

Though Japanese officials say the water being discharged is less radioactive than the water now leaking into the sea , its top concentration of radioactive iodine-131 is 20 becquerels per cubic centimeter , or 200,000 becquerels per kilogram . That 's 10 times the level of radioactivity permitted in food . But since it 's being dumped into the Pacific , it will be quickly diluted , according to Dr. James Cox , a radiation oncologist at Houston 's MD Anderson Cancer Center and a CNN consultant .

Reactors No. 1 and No. 3 , which have lower levels of water , need to be drained as well . Tokyo Electric 's plan is to pump that water to other storage tanks , including some that still need to be set up .

Attempts to fill the 20-centimeter -LRB- 8-inch -RRB- crack outside the No. 2 reactor 's turbine building -- on Saturday by pouring in concrete , then Sunday by using a chemical compound mixed with sawdust and newspaper -- were not successful .

As officials mull other ways to cut off the leak at its source , workers will install a silt fence along a damaged sea wall surrounding the plant , Nishiyama said . The aim of this screening , usually used to halt erosion at construction sites , is to prohibit the spread of radioactive particles into the sea .

Workers also have injected a dye tracer into the water to allow them to track the dispersal of such particles , the spokesman added .

Addressing the issue quickly is critical because officials believe it is a source of alarmingly high radiation levels in seawater near the plant , as well as in nearby groundwater .

Complicating the situation is the fact that , in some cases , authorities do n't even know how much radiation is getting out .

After some high-profile errors , little new information on water , ground and air radiation has been released since Thursday . One reason is that the dosimeters being used do n't go above 1,000 millisieverts per hour , said Junichi Matsumoto , an executive with Tokyo Electric .

Authorities know the water in the cracked concrete shaft is emitting at least that much radiation -- which equates , at a minimum , to more than 330 times the dose an average resident of an industrialized country naturally receives in a year .

Plugging the external leak is job one , in order to prevent the outflow of radiation into the Pacific . But it may not be the most difficult , or important , task ahead .

Authorities still have to figure out how the tainted water got into the concrete shaft in the first place . The water had to come from somewhere , potentially traveling across melted-down nuclear fuel in the reactor 's core before somehow reaching the outside .

`` We were assuming and hoping -LRB- that water -RRB- would stay in the containment vessel as vapor after being cooled , '' Nishiyama , the nuclear safety official , said Sunday . `` However , it may have flowed into the building , and then the trench . ''

Determining why and how that happened -- and what to do about it -- may be `` exceptionally challenging , '' said physicist James Acton , with the Washington-based Carnegie Endowment think tank . Officials may have to inspect a complex array of pipes inside the dangerous radioactive environment inside the containment buildings .

The state of the Nos. 5 and 6 units is another new problem . Water in their turbine buildings ' basements threatens the power supply for the system used to cool nuclear material in these units ' spent fuel pools , Edano said . This makes it imperative to pump out that water , which will end up into the sea like that from around the Nos. 3 and 4 units .

`` Though those reactors are stable at the moment , the growing water level in the turbine houses may disturb their stability , '' he said .

The effort to keep the Nos. 1 , 2 and 3 reactor cores and spent fuel pools cool took a step forward Sunday , when the electricity source powering those three units ' cooling systems was switched from a temporary diesel generator to a more permanent , external power supply , according to the International Atomic Energy Agency 's website .

Authorities hope this step , as well as preventing damage to the Nos. 5 and 6 units ' power supply , will help to minimize the prospect of any more radiation that might contaminate tap water or food .

Farmers have pushed for lower standards on radiation in food , calling them unnecessarily stringent . On Monday , Edano said these limits would not change , even as he outlined a process in which sales restrictions on certain crops , in certain areas , would be lifted if they test safe three times in a row .

CNN 's Matt Smith , Tsukushi Ikeda , Yoko Wakatsuki , Junko Ogura , Midori Nakata , Susan Olson and Martin Savidge contributed to this report

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NEW : Experts say the release will not likely cause lasting environmental harm

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Water from a treatment facility and around four reactors is being dumped

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The official calls the step `` unavoidable , '' saying it 's needed to ensure safety

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Basement water threatens the `` stability '' of the cooling systems of Nos. 5 and 6 units